In recent years, hydrogen production by decomposition of water using solar energy, and metal-air secondary batteries have been actively developed. In particular, the metal-air secondary batteries are expected as novel high-capacity storage batteries that will replace the currently mainstream lithium-ion secondary batteries, and research and development is being conducted to promote the widespread use thereof by about 2030. However, at present, in order to put them to practical application, various problems to be solved have arisen, and one example thereof is the development of a highly active catalyst for an oxygen evolution reaction (OER).
It is known that a large overvoltage is typically generated in the OER, and therefore, sufficient charging and discharging efficiency has not been obtained at present. Therefore, a highly active OER catalyst that greatly affects the voltage during charging urgently needs to be developed. As a catalyst that is highly active against the OER, precious metal catalysts such as Pt, IrO2, and RuO2 (2.5 to 3.0 mA/cm2@1.6 V vs RHE, in a 0.1 mol dm−3 NaOH aqueous solution) (Non-Patent Document 1) are generally known. However, since precious metals are expensive and the reserves are small, the development of an OER catalyst containing no precious metal is required in order to promote widespread use.
Recently, as a non-precious metal OER catalyst containing no precious metal, a perovskite-type transition metal oxide ABO3 has been reported. The perovskite-type oxide includes a transition metal at the B site and has an octahedral structure in which the transition metal binds to six oxygen atoms. Recently, it is reported that the number of eg electrons in the transition metal at the B site is associated with OER activity, and that La0.5Ca0.5CoO3-δ and the like in which the number of eg electrons is close to 1 are highly active (1.5 mA/cm2@1.6 V vs RHE, in a 0.1 mol dm−3 NaOH aqueous solution) (Non-Patent Document 2).
Non-Patent Document 1: Y. Lee, et al., J. Phys. Chem. Lett. 2012, 3, 399.
Non-Patent Document 2: Suntivich, et al., Science 2011, 334, 1383.    Non-Patent Documents 1 and 2 are expressly incorporated herein by reference in their entireties.